full wave bridge rectifier volts change question

I have a full wave bridge rectifier that changes volt amounts outputted when I hit a button in the circuit. Maybe I don't understand them properly. I thought their output stays at a fixed DC volt like a battery. Here is what I'm testing.

Measuring volts with voltmeter at point 13 (+) and point 2 (-). Point 2 is ground. Point 13 is the + output of the full wave bridge rectifier coming from that transformer. I highlighted these points in yellow. Initial reading is around +30V. When I press PLAY (highlighted green button) the volts drop to about +27V. Any ideas? Please enlighten me.

Can't change the power supply. I have to use that transformer and rectifier setup.

If you look at the original schematic, my new circuit is similar to theirs. I'm using that power and a bunch of switches, buttons, and relays. My circuit is just a little smaller. What I don't understand is how did they setup their total schematic using unregulated power? Wouldn't their volts be changing every time they throw a switch? I measured the old machine and somehow their volts aren't changing at the switches, but my new circuits are.

That is why I wanted to take the 30V DC (that is straight from the rectifier with no load) and regulate it down to 27V and then hook everything up. This way the 27V will not change depending on what my switches and buttons are doing. Correct?